# A New Spectral Feature on the Trailing Hemisphere of Europa at 3.78 microns

## TL;DR

This study presents new high-quality spectra of Europa's surface, revealing a unique 3.78 micron feature linked to radiolytic processes, challenging previous SO2 interpretations and suggesting alternative surface compositions.

## Contribution

First high-quality L-band spectra of Europa extending beyond 4 microns, identifying a new spectral feature and questioning prior SO2 detection interpretations.

## Key findings

- Detected a 3.78 micron spectral feature on Europa's trailing hemisphere.
- The feature's distribution suggests a radiolytic origin from electron or ion bombardment.
- The feature is not explained by SO2 frost, contrary to previous hypotheses.

## Abstract

We present hemispherically resolved spectra of the surface of Europa from ~3.1--4.13 microns, which we obtained using the near infrared spectrometer NIRSPEC on the Keck II telescope. These include the first high-quality L-band spectra of the surface to extend beyond 4 microns. In our data we identify a previously unseen spectral feature at 3.78 microns on the trailing hemisphere. The longitudinal distribution of the feature is consistent with that of a radiolytic product created by electron or Iogenic ion bombardment. This feature is coincident with an absorption feature of SO2 frost seen in both laboratory spectra and spectra of Io. However, the corresponding, typically stronger 4.07 micron feature of SO2 frost is absent from our data. This result is contrary to the suggested detection of SO2 at 4.05 microns in Galileo NIMS data of the trailing hemisphere, which was severely affected by radiation noise. We use simple spectral modeling to argue that the 3.78 micron feature is not easily explained by the presence of SO2 frost on the surface. We explore alternative explanations and discuss other potential candidate species.